In the process of electrorefining, it is highly desirable to deposit metals in the form of small individual pieces rather than as large cathodes, since smaller pieces can be more readily charged into melting furnaces or electroplating baskets. When, in conventional refining processes, elemental metals are produced as large cathodes, e.g., 70.times.90 centimeters, it is generally advantageous and in many cases necessary to divide these large cathodes into smaller pieces, e.g., 5.times.5 cm. Large cathodes are generally reduced in size by shearing; however, this operation entails considerable labor expense and wear of the shear blade necessitates frequent replacement.
In order to overcome the necessity for a shearing operation, several methods have been devised for producing cathodes in the form of small pieces. For example, in the production of nickel as described in U.S. Pat. No. 3,577,330, a permanent cathode mandrel, or workpiece, was designed which contains conductive islands on its surfaces. The conductive islands are defined by means of interconnecting areas of non-conductive materials such as paint, varnish, lacquer and tape. U.S. Pat. No. 3,668,081 is concerned with a more advanced mandrel made of chromium plated stainless steel masked with a thermosetting epoxy ink or paint that is silk-screen-printed upon the surface to provide the desired masking.
The aforedescribed mandrels are currently in use for the production of electrolytic nickel. For refining this metal as well as cobalt and iron, the mandrels work effectively and may be used several times without requiring refurbishing. However, in the process of refining a metal such as copper, the electrolyte is considerably more corrosive than that used for plating of nickel, cobalt and iron. As a consequence, the aforedescribed mandrels have been found to have limited life in copper refining electrolytes due to deterioration of the adhesive bond at the junction of metal and coating. Such deterioration is believed to be associated with difference in coefficients of thermal expansion, corrosive attack of the metal mandrel, abrasion during use and dissolution and/or hydrolyzation of the masking substance. Deterioration can occur during a single use in a copper electrolyte and results in the formation of undesirably large and irregularly shaped cathode pieces unsuitable for the marketplace. Also, when repair of the epoxy layer of a prior art mandrel is required, the application of heat can cause undesirable overaging and brittleness in the epoxy layer immediately adjacent to the repair.
Similarly in electroforming, or fabrication of articles by electrodeposition, a need exists for a mandrel that will be readily separable from the electrodeposit and will resist tearing and penetration of the electrolyte between the conductive and non-conductive portions of the mandrel. Furthermore, it is highly desirable that the mask itself resist tearing during removal of the electroform so that the mandrel can be used many times. In addition, the availability of a conductive flexible polymeric mandrel would be of great advantage in the art to circumvent the necessity for coating a non-conductive workpiece with a conductive substance, e.g., graphite, silver, etc., prior to plating.